1. Field of the Invention
The present invention relates generally to a technique for removing contaminants from a contaminated substrate. More particularly, the present invention relates to a technique for thermally separating contaminants from a contaminated substrate by heating the contaminated substrate so as to volatilize the contaminants into a vapor that is later removed to allow for the recovery of the contaminants.
2. Background Art
Increasing attention has been given to public health consequences resulting from the introduction of industrial wastes, e.g., halogenated and non-halogenated organic compounds, into the environment. Incident with this attention have come governmental regulations that have been put in place to mandate the removal of such industrial wastes to maximum permissible residual levels in the soils and other matrices of former disposal sites.
Traditionally, clean-up of disposal sites involved the procedure of removing contaminated soil or material to a designated secure land fill area. However, the number and volume of designated land fill areas have been greatly reduced, and therefore, there is a growing need to sanitize soils and other matrices with efficient and economical treatment processes.
In one response to this need, portable incineration systems have been proposed. For example, U.S. Pat. No. 4,667,609 issued to Hardison et al. discloses a mobile apparatus for infrared heating of soils contaminated by various hydrocarbons. However, in such incineration systems, the heating step is typically carried out to the point of complete combustion. Hence, operation of such a system is prone to preclusion by governmental regulations that are extremely stringent with respect to the allowable levels of output gases and the like from incineration systems. Furthermore, incineration systems do not allow for the recovery of valuable hydrocarbons in the contaminated substrate.
In another response to the need for efficient and economical sanitization processes, U.S. Pat. No. 4,864,942 issued to Fochtman et al., for example, discloses a method for removing organic compounds, e.g., PCBs, from soils by volatizing the organic compounds at temperatures well below that which are used in the incineration systems described above. Generally, these non-incineration temperatures typically do not exceed 1200 degrees Fahrenheit. However, the time needed for such treatment, i.e., treatment without combustion, to result in complete volatilization of contaminants is often undesirably long. For example, in some cases, such volatilization may take an hour or more, and in some other cases, such as with respect to mercury, volatilization may not occur at all.
In light of the needs to maximize heat transfer to a contaminated substrate using temperatures below combustion temperatures, U.S. Pat. No. 6,399,851 discloses a thermal phase separation unit that heats a contaminated substrate to a temperature effective to volatize contaminants in the contaminated substrate but below combustion temperatures. As shown in FIGS. 3 and 5 of U.S. Pat. No. 6,399,851, the thermal phase separation unit includes a suspended air-tight extraction, or processing, chamber having two troughs arranged in a “kidney-shaped” configuration and equipped with rotating augers that move the substrate through the extraction chamber as the substrate is indirectly heated by a means for heating the extraction chamber.